About Optics & Photonics TopicsOSA Publishing developed the Optics and Photonics Topics to help organize its diverse content more accurately by topic area. This topic browser contains over 2400 terms and is organized in a three-level hierarchy. Read more.

Topics can be refined further in the search results. The Topic facet will reveal the high-level topics associated with the articles returned in the search results.

Abstract

We report the imaging of sub-diffraction limited features using an optical probe generated by focusing a round spot at one wavelength, λ1 = 405nm, and a ring-shaped spot at a second wavelength, λ2 = 532nm, onto a thin photochromic layer that coats the nanostructures. Illumination at λ2 turns the photochromic layer opaque to λ1 everywhere except at the centre of the ring, where the illumination at λ1 penetrates and probes the underlying nanostructure. We confirm that this optically confined probe increases image contrast and is able to resolve features smaller than the far-field diffraction limit. Furthermore, by using an array of dual-wavelength diffractive microlenses, we demonstrate the feasibility of parallelizing this approach. Compared to previous approaches, our technique is not limited to fluorescence imaging.

Figures (8)

Absorbance modulation for optical nanoscopy. (A) The sample to be imaged (depicted as absorber features on a glass substrate) is over-coated with a layer of photochromic molecules. These photochromic molecules turn transparent when illuminated by λ1 and recover their original opaque state when illuminated by λ2. When this overlayer is illuminated by a ring-shaped spot at λ2 coincident with a round spot at λ1, the photostationary state results in a narrow transparent region in the vicinity of the λ2 node. Light at λ1 penetrates this region forming a spatially confined probe. Light at λ2 is filtered out beyond the sample, and the signal from the transmitted light at λ1 is collected. (B) In our experiments we used the shown azobenzene polymer as the photochromic layer. These molecules switch from the trans to the cis isomer when illuminated by λ1 = 405nm and recover to the original trans form when illuminated by λ2 = 532nm.

Schematic of the absorbance-modulation imaging (AMI) system. The inset on the left shows an optical image of the microlenses of various numerical apertures (NAs). The scanning stage has a clear aperture and the transmitted light is collected. The microlenses are spaced such that the signals are read out in parallel. The λ2 beam is not shown past the sample for clarity.

Absorbance-modulation imaging of periodic metal lines. (A) Transmitted-light images of a 500nm-period grating in a 50nm-thick layer of chromium on glass, taken with only λ1 = 405nm (top) and with both λ1 = 405nm and λ2 = 532nm (bottom). (B) Average linescans through the images in (A). The image contrast is increased by a factor of ~2 due to absorbance modulation. The sample was overcoated with 220nm of the azobenzene polymer. The numerical aperture (NA) of the microlens was 0.83. The signals were collected in transmission. The peak focal intensities at λ1 and λ2 incident on the sample were 34.6W/m2 and 0.4W/m2, respectively.

(A) Schematic of parallel imaging. The array of microlenses is illuminated by both wavelengths. Each microlens forms a focused node-spot pair that probes the sample. The transmitted light at λ1 (the signal) is collected on a CCD. Since the focii of the microlenses are far apart, the signals are spatially separated on the CCD camera as indicated on the schematic on the right. (B)-(D) Transmission images of 10nm gold nanoparticles randomly dispersed on a glass slide. Top row shows images taken with only λ1 = 405nm. Bottom row shows images of the corresponding regions on the sample taken with both λ1 = 405nm and λ2 = 532nm. The images in each row were acquired by different microlenses in parallel. The images were taken with microlenses of NA = 0.83 in (B) and (C), and NA = 0.55 in (D). There was a slight shift in the sample between the scans shown in the top and bottom rows. The signal levels in each image are normalized. The peak focal intensities at λ1 and λ2 incident on the sample were 34.6W/m2 and 334W/m2 in (B) and (C), and 3.4W/m2 and 37.2W/m2 in (D), respectively. The step-size for the top row during image acquisition was 100nm.

(A) Schematic of sample, a glass slide with gold nanoparticles randomly dispersed on the surface. The slide was coated with 220nm of the azobenzene polymer. Note that the signal is composed of light transmitted through the gaps between the nanoparticles. (B)-(C) Images of 100nm gold nanoparticles taken by a microlens of NA = 0.7 with only λ1 = 405nm (B) and with both λ1 = 405nm and λ2 = 532nm (C). The peak focal intensities at λ1 and λ2 incident on the sample were 21.23W/m2 in (B) and 13.36W/m2 and 373W/m2 in (C), respectively. (D)-(F) Images of 10nm gold nanoparticles dispersed on a glass slide taken with a microlens of NA = 0.55. The peak focal intensities at λ1 and λ2 incident on the sample were 2.95W/m2 and 50W/m2 in (D), 2.16W/m2 and 50 W/m2 in (E), and 1.42W/m2 and 50W/m2 in (F), respectively. As the ratio of the intensity at λ2 to that at λ1 increases (from (D) to (F)), nanoscale structural details are revealed. (G) Magnified image of the area within the white square in (F). Some possible geometries of nanoparticle clusters are outlined with black dashed lines. Two (possibly) single nanoparticles spaced by a distance of 40nm is clearly resolved. (H) Signal cross-sections through white lines shown in (D)-(F). When the intensity ratio of λ2 to λ1 is sufficiently high, structures that are ~50nm apart are revealed. A bicubic interpolation from the raw image data was used in (G) and (H).

Transient behaviour of light in photochromic layer, when illuminated from the top by a focused spot at λ1 and a focused node at λ2. (A) Normalized intensity distribution of λ1 inside the photochromic layer for different instances of time after both sources are turned on. As the beam propagates through the material, it also spreads in the lateral direction, making the subsequent “probe” larger. (B) Cross-section of normalized intensity distributions at the bottom of the photochromic layer at the same three instances of time shown in A. The width of the mainlobe increases with time as do the intensities of the sidelobes. The peak intensity also increases with time although it is not evident in the normalized plots. The images are only intended for illustration and are not actual simulations.

Effect of delay between start of illumination and signal acquisition. The figure shows images of 100nm gold nanoparticles dispersed on a glass slide imaged with microlenses of NA = 0.83 (A) and NA = 0.7 (B). A delay of 1 second was used for the top images, while a delay of 5 seconds was used for the bottom images. The peak intensities at λ1 and λ2 were (A) I1peak = 0.38W/m2, I2peak = 0.05W/m2, and (B) I1peak = 0.25W/m2, I2peak = 0.04W/m2, respectively. For longer delay times, the size of the probe beam (at λ1) increases as the azobenzene layer evolves to its photostationary state. This was confirmed by the poorer contrast in the images with longer delays (bottom row). Note that the images in each row were acquired at the same time by two different microlenses, further demonstrating the potential for parallelism. The step-size for these images was 100nm and no interpolation was used.

Images of the same region of a resolution-test structure taken with no delay (left) and with a delay of 0.5s (right). The images were acquired with a microlens of NA = 0.83. The peak intensities of λ1 and λ2 were 34.6W/m2 and 334W/m2, respectively. Higher intensities force the azobenzene layer to reach the photostationary state much faster, and the effect of delay on the images vanishes. The step-size for these images was 250nm and no data interpolation was used.